This invention relates to ophthalmologic methods and systems, and in particular, to methods and systems for detecting damage to the retina, such as that caused by glaucoma.
The retina is a highly organized but complex neurosensory tissue that processes visual information and transmits it to the brain along the optic nerve. The optic nerve, which connects the eye to the brain, is predominantly a bundle of axons, or fiber-like projections of neuronal cells called retinal ganglion cells. These retinal ganglion cell axons fan out along the superficial aspect of the retina in arcuate bundles. When light stimulates photoreceptors, which are located under these arcuate bundles, a signal is transmitted, via a complicated array of intervening neuronal cells, to the retinal ganglion cell body. The retinal ganglion cell axons then relay this signal through an exit site, referred to as the xe2x80x9coptic nerve headxe2x80x9d, in the posterior wall of the eye. The optic nerve head corresponds functionally to the blind spot in vision because, at this location, there are no underlying photoreceptor cells.
Glaucoma is a disease of the retinal ganglion cell bodies and their axons. In a patient having glaucoma, the retinal ganglion cells slowly lose their ability to transmit nerve impulses. As a result, vision diminishes, often so slowly that a patient afflicted with this disease does not notice the degradation in vision until significant damage has occurred. It is in this insidious manner that glaucoma robs the patient of sight. When detected early enough, glaucoma can be managed. However, because glaucoma has few overt symptoms, it is difficult to detect early.
One approach to testing for glaucoma is to use a tonometer to measure intra-ocular pressure. This test is based on the notion that high intra-ocular pressure can damage the retinal ganglion layer. However, in practice, intra-ocular pressure has not proven to be a reliable indicator for glaucoma.
A more reliable test for glaucoma is a visual field test in which light is directed to various portions of the retina. By asking the patient whether he sees the light, one can map the sensitivity of the retina. Because the field vision test measures optic nerve function more directly, it is a more accurate indicator of glaucoma than the tonometric test. However, the visual field test is a time-consuming test that requires expensive equipment operated by trained personnel. Furthermore, the visual field test assesses all components of the visual system, from the tear film to the occipital cortex in the brain. It is not testing the function of retinal ganglion cells specifically. As a result, it can be difficult to distinguish pathology of retinal ganglion cells from pathologies of other components of the visual system.
The invention is based on the recognition that under certain circumstances, a patient having normal retinal function will perceive an entoptic signal. This entoptic signal, which most commonly appears to the patient as a blue arc, is believed to originate in the retinal ganglion cell axons. A patient""s inability to perceive this entoptic signal is correlated with the likelihood that the patient""s retina has experienced damage, perhaps from early stages of glaucoma. This correlation is exploited in a simple and inexpensive test for evaluating the likelihood that a patient has glaucoma.
A method according to the invention includes selecting a test site on a retina of the patient and stimulating that test site to cause the generation of an entoptic signal. The entoptic signal need not be generated at the test site but can be generated sewhere in the eye. The entoptic signal generated as a result of the stimulus is then detected. In some practices of the invention, the entoptic signal is detected by accepting an input from the patient that indicates whether or not the patient has perceived any visual manifestation of the entoptic signal. In other practices of the invention, entoptic signal is detected by obtaining an objective measurement with a device for measurement of electromagnetic fields or waves.
In some practices of the invention, the test site is selected to be temporal, or peripheral to the patient""s optic nerve head. In other practices of the invention, selecting a test site includes providing a target upon which the patient is to fixate. In those cases, stimulating the test site includes displaying, to the patient, a test figure peripheral to the target.
The test site can be stimulated by illuminating it with a test figure. Alternatively, the test site can be stimulated by displaying a test figure to the patient on, for example, a computer monitor or a specialized display device. In either case, the test figure can periodically be made to flash on and off. The frequency and duty cycle with which the test figure flashes is selected to enhance the patient""s visual perception of the entoptic signal.
These and other features of the invention will be apparent from the following figures, in which: